Apparatus and method for determining vehicle wheel alignment measurements from three dimensional wheel positions and orientations

ABSTRACT

Apparatus and method for determining the alignment positions and orientations of vehicle wheels includes optical targets mounted on the wheels and optical targets mounted in a fixed relationship with respect to the surface on which the wheels are disposed. Video cameras are used to obtain images of the various optical targets and a computer is responsive to the images of the targets to determine values of wheel alignment parameters of the vehicle relative to said surface on which the vehicle wheels roll. The surface on which the wheels are disposed may be an automotive lift, and apparatus is disclosed for keeping the optical targets in the same position in the field of view of the camera(s) whether the lift is in its rest or in an elevated or reclined position.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation application Ser. No. 08/651,766 filed on May 22,1996, now U.S. Pat. No. 5,675,515, which is a continuation-in-part ofU.S. application Ser. No. 08/580,465, filed Dec. 28, 1995, now U.S. Pat.No. 5,724,128.

BACKGROUND OF THE INVENTION

The present invention relates to vehicle wheel alignment, and moreparticularly to vehicle wheel alignment systems which measure thelocations and orientations of the vehicle wheels in a three dimensionalcoordinate system.

Various systems have been designed to determine vehicle wheel alignmentangles. For example, U.S. Pat. No. Re 33,144 to Hunter and January andU.S. Pat. No. 4,319,838 to Grossman and January each describe a wheelalignment system which uses electro-optical transducers to determine thetoe alignment angles of a vehicle. FIG. 2 of each of these patents showssix angle transducers carried by support assemblies which are mounted tothe vehicle wheels. FIG. 4 of Re 33,144 and FIG. 9 of U.S. Pat. No.4,319,838 show the geometry of this arrangement and illustrate the sixangles which are directly measured. These patents further describe (seeRe 33,144 col. 7 lines 26-39, and 4,319,838 col. 8 line 63 to col. 9line 12) how the toe alignment angles are computed from the anglesdirectly measured by the angle transducers.

U.S. Pat. No. 4,879,670 to Colarelli describes a gravity-referencedinclinometer. FIG. 3 of 4,879,670 illustrates the mounting of such aninclinometer to a vehicle wheel for measuring the camber of the wheel.The use of gravity-referenced inclinometers to measure camber isconventional, and assumes the vehicle rests while being measured on asurface which is both flat and level.

SAE Publication 850219, titled "Steering Geometry and CasterMeasurement", by January, derives and discusses the procedures andmethods by which toe and camber alignment transducers are used todetermine the caster and steering axis inclination (SAI) of a vehicle.The procedures described therein are the industry standard.

Equipment of this general type and using the apparatus and methodsenumerated above has been used world-wide for many years. Such equipmentis capable of determining the camber, caster, and pointing or "toe"alignment angles of the wheels relative to one or more appropriatereference axes, and is sufficient to allow proper adjustment of thealignment so as to reduce tire wear and provide for safe handling. It isbelieved, however, that such equipment could be improved.

U.S. Pat. No. 5,488,472 to January advances the art further bydescribing the use of conventional toe transducers which, whileoperating in cooperative pairs, have the additional capability ofmeasuring the distances, each relative to the other. FIG. 7 of U.S. Pat.No. 5,488,472 illustrates the use of these "range and bearing"measurements to determine the coordinates and orientations of thesensors and wheels in a two dimensional coordinate system.

FIGS. 8 through 11 of U.S. Pat. No. 5,488,472 illustrate the seriousnessof the central problem in measuring the individual toe alignments ofvehicle wheels, namely that individual toe of a vehicle wheel is definedto be relative to a longitudinal reference axis, but the definition ofthat reference axis may not be arbitrarily chosen by the designer ofsuch equipment. In general, two such reference axes are used, thedefinitions of which were standardized long ago in the automotiveservice industry.

The individual rear toe alignment angles are defined to be relative to areference axis commonly known as the "geometric centerline". This line,illustrated and labeled "CL" in FIG. 9B of U.S. Pat. No. 5,488,472, ispractically determined as the bisector of the angle formed by thelongitudinal lines of sight of the toe transducers. These lines of sightare illustrated and labeled 48 and 50 in FIG. 6 of U.S. Pat. No.5,488,472. A key aspect of this definition is that the forward endpointsof these longitudinal lines of sight are remarkably insensitive to smallchanges in the steering directions of the front wheels, which means thatthe individual rear toe alignment measurements are similarly insensitiveto the steering of the front wheels.

The layman's definition of the geometric centerline is the line joiningtwo points, one lying halfway between the front wheels and the otherlying halfway between the rear wheels. This line very closelyapproximates the centerline described above and is very easy tovisualize.

The individual front toe alignment angles are defined to be relative toa reference axis commonly known as the "thrust line". This line,illustrated and labeled "TL" in FIG. 9A of U.S. Pat. No. 5,488,472, ispractically determined as the bisector of the angle formed by thereference axes of the rear longitudinal toe transducers. These referenceaxes are illustrated and labeled 59 and 60 in FIG. 9A of U.S. Pat. No.5,488,472. A key aspect of this definition is that the thrust line isdetermined as the net pointing direction of the rear wheels, which meansthat the individual front toe alignment measurements are intentionallysensitive to the toe alignment of the rear wheels.

The layman's definition of the thrust line is the line which bisects theangle formed by the planes of rotation of the rear wheels. This line isvery easy to visualize.

There are great practical advantages in determining toe alignmentrelative to these reference axes. Firstly, the toe adjustment of therear wheels can be accomplished with the front wheels steered onlyapproximately straight ahead. Secondly, the thrust line thus determinedis approximately the line down which the center of the rear axle travelswhen the vehicle moves in a straight line, and this line is made topoint approximately down the centers of the front and rear axles.Thirdly, the toe adjustment of the front wheels can be accomplished withthe steering wheel held straight such that the front toe measurementsare symmetric about the thrust line, thereby insuring that the steeringwheel is straight when the vehicle moves in a straight line. Fourthly,vehicle manufacturers have long provided toe alignment specificationswhich are relative to these reference axes. Any vehicle alignment systemwhich defines toe alignment relative to other axes will not be able tocorrectly align the vehicle as specified by the vehicle manufacturers.

The disclosure of U.S. Pat. No. 5,488,472 illustrates that determiningthe two dimensional coordinates of the vehicle wheels does not providegreater ability to determine the toe alignments of the wheels relativeto the appropriate reference axes of the vehicle. The onlydeterminations of these reference axes which are practical to use arethe same as those provided by transducers which measure angles but donot additionally measure distances.

U.S. Pat. Nos. 4,745,469 and 4,899,218, both to Waldecker et al.,describe what is commonly known as an "external reference aligner". U.S.Pat. No. 4,899,218 is a continuation of U.S. Pat. No. 4,745,469, andcontains no new disclosure. FIGS. 3 through 6 of these patents show howlasers are used to illuminate the tires and video cameras are used toexamine images of the sidewalls. These patents further describe how"machine vision techniques" are used to examine the images and determinethe distances between the cameras and certain locations on thesidewalls, thereby allowing a determination of the locations andorientations of the wheels in a coordinate system which is relative tothe cameras.

Unfortunately, both U.S. Pat. Nos. 4,745,469 and 4,899,218 are woefullydeficient in describing how a determination is made of the toe alignmentof the wheels relative to the appropriate reference axes of the vehicle.The need for this is discussed in U.S. Pat No. 4,745,469, col. 2, lines19-24:

"This wheel position information can be combined with similarly measureddata defining the vehicle center line or other desired references andthe complete vehicle alignment geometry can be analyzed and dynamicallydisplayed on a meter or the like to guide an operator in adjusting orsetting the wheel alignment."

Beyond this, U.S. Pat. No. 4,745,469 has no disclosure of how the toemeasurements are determined relative to the vehicle center line. Areference line L is defined in FIG. 2 and discussed in col. 4, lines13-23:

"The spatial position of the wheel . . . may be defined in relation to .. . a longitudinal line L which passes through two fixed points on thevehicle. For convenience, longitudinal line L is shown extending throughthe front and rear bolts 24 and 26 which attach the control arm to thevehicle chassis. . . . In FIG. 1, a second longitudinal line L' is drawnparallel to longitudinal line L so that it passes through wheel axis A.The angle between longitudinal line L' and the center plane Cestablishes the toe-in of the wheel."

This is not believed to be a useful definition for a reference axis fordetermining toe alignment. Firstly, not all vehicles have an uppercontrol arm with mounting bolts as described. Secondly, vehicles whichhave such upper control arms have one for the left front wheel and onefor the right front wheel, and thus would have two different such linesL, even though left front toe and right front toe should be determinedrelative to the same reference axis. Thirdly, vehicles which mount anupper control arm in the manner illustrated in FIG. 2 of U.S. Pat. No.4,745,469 commonly use shims, eccentric cams, or elongated slots to movethese mounting bolts, thereby adjusting camber and/or caster of theaffected wheel. Fourthly, no vehicle manufacturer specifies toealignment relative to such an axis.

U.S. Pat. No. 4,745,469 describes determining the toe alignment of awheel in yet another way in col. 16, lines 10-25:

"Once two points in real space have been found, corresponding to twopoints along a horizontal line on the tire, the toe-in is easilycomputed using trigonometry. Referring to FIG. 21, the left and rightsensor modules 36 are illustrated together with a portion of the tire12. Each sensor is a predetermined distance from a reference point REF.The distances are designed Y_(L) and Y_(R). The spacing between the leftand right data points P_(L) and P_(R) is therefore Y_(L) +Y_(R). Thereal space position of points P_(L) and P_(R) in the Z direction are themeasured values Z_(L) and Z_(R) determined by the conversion from imagespace data to real space data. If the points P_(L) and P_(R) do not havethe same Z coordinates, then there is a non-zero toe angle. This angleis determined by trigonometry as the arc tangent of the difference(Z_(R) -Z_(L)) divided by the sum (Y_(R) +Y_(L))."

This definition would have individual toe measured relative to the videocameras, which would provide the proper value only if the vehicle weresquarely aligned relative to the cameras, which is highly impractical.

It is readily apparent from U.S. Pat. Nos. 4,745,469 and 4,899,218, inlight of U.S. Pat. No. 5,488,472, that it is not sufficient merely todetermine the locations and orientations of the vehicle wheels in athree dimensional coordinate system. Proper attention must be paid todetermining the toe alignment of the wheels relative to the appropriatereference axes.

German Patent DE 29 48 573 A1, assigned to Siemens AG, describes the useof video cameras to determine the locations and orientations of thewheels of a vehicle. On each side of the vehicle, a single camera ismoved to multiple positions to view the vehicle wheels. Alternatively, asingle fixed camera is used at each side in conjunction with movablemirrors, or multiple cameras are used. The system examines the imagesthus viewed of the wheels of the vehicle to determine the locations andorientations of the wheels, from which the wheel alignment parametersare determined.

This patent provides scant details concerning how the wheel alignmentparameters are determined from measurements made by the video cameras.For example

"These parameters are given essentially by the spatial position of thewheel suspension (steering axle) relative to the wheel plane or tovertical or horizontal reference planes." and

". . . the spatial positions of the wheel plane and of the steering axleand, from the appropriate data as well as the stored positional data ofthe video camera tube and the wheel dimensions, the wheel axle andsteering geometry data are determined electronically on the basis ofconsecutively obtained measurement results, taking into considerationknown mathematical relationships." and

"Because of the conical section geometry as well as the circle-ellipseaffinity, the axis of the body of rotation and, with that, the spatialaxis of the wheel suspension, that is, the steering axis as well as thespatial position of the wheel plane, can be determined from thedifferent positions of the wheel and the data of the wheel axles and thesteering geometry calculated from the mutual allocation or theallocation to the vertical or horizontal reference planes."

This disclosure also fails to describe how individual toe alignmentmeasurements are determined from the wheel position data. Note that thispatent application was filed Dec. 3, 1979, a time when four wheelvehicle alignment embodying thrust line alignment of the rear wheels wasin its relative infancy.

European Patent Application PCT/US93/08333, filed in the name of Jacksonand published under the Patent Cooperation Treaty as WO 94/05969(hereinafter referred to as WO document 94/05969), describes the use ofa video camera having one or more defined fields of view to view opticaltargets of known configurations which are mounted to the vehicle wheels.Through the use of sophisticated image recognition methods, the threedimensional coordinates and orientations of the vehicle wheels and theircorresponding axes of rotation are determined. The wheel alignmentparameters are determined from these coordinates and orientations.

This application treats the determination of individual toe alignmentand individual camber alignment sketchily. See, for example, page 7,lines 28-34:

". . . the processor relates the dimensions of certain known geometricelements of the target with the dimensions of corresponding elements inthe perspective image and by performing certain trigonometriccalculations (or by any other suitable mathematical or numericalmethods), calculates the alignment of the wheels of the vehicle."

See also page 25, lines 1-4:

"As has been described above, once the location of the target planes onthe wheels is known, by rotating the wheels, the axis of rotation of thewheels can be determined, and from there, the alignment of the wheels."

A hint as to how this is performed is found on page 40, lines 2-8:

"So, for example, the apparatus could define a reference point for eachwheel with the referent point being located at, say, the intersection ofthe axis of rotation of the wheel, with that wheel. These points canthen be processed to define an approximately horizontal reference plane,relative to which the alignment of the wheels can be calculated."

Although there is considerable disclosure in the WO document 94/05969concerning how to determine the coordinates and orientations of thewheels and their axes of rotation in a three dimensional coordinatesystem, there is no disclosure which explains how the toe alignment orcamber alignment is determined from those coordinates and orientations.As has been made clear above, this is not a trivial subject.

There is further a fundamental flaw in this methodology. Quite simply,the three-dimensional coordinates of the vehicle wheels, and the axesabout which they rotate, are not sufficient to properly determine thewheel alignment parameters of the vehicle unless the plane representingthe surface on which the wheels roll is also known in that coordinatesystem. Firstly, the camber, caster, SAI, and toe alignment parametersof the wheels are defined relative to this plane. Secondly, the WOdocument 94/05969 does not provide any method or apparatus fordetermining where this plane is in its coordinate system, even in themost general terms. The subject is not discussed therein. As will bemade more apparent presently, handling, tire wear, stability, and safetyissues are compromised thereby.

There exists a clear need for apparatus and methods which allow a properdetermination of the alignment of the vehicle wheels, per theirconventional and accepted definitions, using measurements of the threedimensional coordinates of the vehicle wheels, the axes about which theyrotate, and the plane on which they roll.

In addition to the above-mentioned drawbacks, proper alignment usingvideo systems is critically dependent upon accurate determination of thepositions of the targets in the field of view. In those instances whereall the targets move vertically in a uniform amount, that movement maybe misinterpreted as one or more changes in alignment angles.Algebraically compensating for such motion in the field of view is not atrivial problem.

SUMMARY OF THE INVENTION

Among the various objects and features of the present invention may benoted the provision of an improved wheel alignment system whichdetermines the camber, caster, toe, and other alignment parameters ofthe vehicle wheels, relative to the surface on which the wheels roll,per their conventional and accepted industry definitions.

A second object is the provision of such a system which determines thealignment parameters of the vehicle wheels from measurements of thecoordinates and orientations of the wheels and their corresponding axesof rotation in a three dimensional coordinate system, as the alignmentrelates to the flat surface on which the wheels roll.

A third object is the provision of method and apparatus which accuratelycorrects for vertically movement of a vehicle in the field of view ofthe video camera(s) so that the measurements thus determined areaccurate.

Other objects and features will be in part apparent and in part pointedout hereinafter.

Briefly, a wheel alignment apparatus of the present invention determinesthe alignment of the wheels of a vehicle in relation to a verticallymovable surface upon which the vehicle wheels are disposed. Theapparatus includes a first set of predetermined optical targets adaptedto be mounted to wheels of a vehicle, a second set of predeterminedoptical targets disposed in a predetermined geometrical relationshipwith respect to the surface on which the vehicle wheels are disposed,and at least one video camera disposed to receive images of the firstoptical targets and the second optical targets, each of the at least onevideo cameras having a field of view. A computer is operativelyconnected to the camera and is responsive to the images of the first setof targets and to the images of the second set of targets to determinevalues of wheel alignment parameters of the vehicle relative to thesurface on which the vehicle wheels are disposed. An elevating mechanismis operatively connected to the at least one video camera to raise andlower the camera along a predetermined path, the elevating mechanismbeing responsive to vertical movement of the vertically movable surfaceto move the camera correspondingly along the predetermined path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a vehicle wheel, showing how the locationand orientation of the wheel and its axis of rotation are represented ina three dimensional coordinate system.

FIG. 2 is a side elevation view of front and rear vehicle wheels whichhave different diameters, thereby illustrating that the plane defined bythe axes of rotation of the wheels does not correspond to the surface onwhich the wheels roll.

FIG. 3 is a front elevation view of left and right vehicle wheels whichhave different diameters, thereby illustrating that the plane defined bythe axes of rotation of the wheels does not correspond to the surface onwhich the wheels roll.

FIG. 4 is an isometric view of the preferred embodiment, illustratinghow video cameras are used to view optical targets which are mounted tothe vehicle wheels and to the lift rack runways on which the vehiclerests.

FIG. 5 is an isometric view of the preferred embodiment, illustratinghow a calibration fixture is used to determine the relationship betweenthe fields of view of the video cameras.

FIG. 6 is an isometric view of a vehicle wheel, illustrating how thelocation and rolling direction of the wheel in a vehicle reference planeare determined.

FIG. 7 is a plan view showing the projections of the locations of fourvehicle wheels onto a vehicle reference plane, along with the linesdescribing the rolling directions of the wheels. The locations andcamber of the wheels are exaggerated for clarity. Total toe of the frontand rear wheels are determined.

FIG. 8 is similar to FIG. 7, illustrating how the reference axis formeasuring rear individual toe is determined from the locations of thefour vehicle wheels.

FIG. 9 is similar to FIG. 7, illustrating how the reference axis formeasuring front individual toe is determined from the pointingdirections of the rear wheels.

FIG. 10 is similar to FIG. 7, illustrating how symmetry angles anddistances of the vehicle wheels are measured from the locations of thewheels.

FIG. 11 is similar to FIG. 4, illustrating the present invention in usein combination with an automotive lift.

FIG. 12 is a diagrammatic view of the system of FIG. 11 showing theinterconnection of the various parts of the system.

Similar reference characters indicate similar parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is preferred that the present invention be embodied in a computercontrolled vehicle wheel alignment system, as is usual and customary inthe art. Most modern wheel alignment systems are built using anoff-the-shelf IBM compatible personal computer (PC) which is connectedto the requisite alignment sensors through the built-in serial ports ofthe PC or through custom designed hardware.

As will be discussed in more detail presently, the sensors of thepresent invention consist of a pair of video cameras which are made toview optical targets mounted to the vehicle wheels. This is very similarto WO document 94/05969 (discussed previously), the full disclosure ofwhich is incorporated herein by reference.

FIG. 1 illustrates the purpose of the video cameras, which is todetermine the coordinates and orientations of the vehicle wheels, andthe axes about which they roll, in a three dimensional coordinatesystem. In FIG. 1, a vehicle wheel 10 rotates about its axis of rotation11. The location 12 of the wheel 10 is defined to be the intersection ofthe axis of rotation 11 with the plane of rotation 13 which representsthe wheel. This plane of rotation 13 is most conveniently visualized asthe approximate outer edge of the wheel rim, and is perpendicular to theaxis of rotation 11.

The location of the wheel 10 is described by the coordinates X_(W),Y_(W), and Z_(W) in a three dimension coordinate system havingorthogonal axes X, Y, and Z. The orientation of the wheel 10 isdescribed by the unit vector U_(W), which points from the location 12 ofthe wheel 10 outwardly along the axis of rotation 11, and by the planeof rotation 13.

Vector U_(W) may be represented in many forms. Perhaps the mostconvenient are in terms of its "direction numbers", which are thedistances along the axes X_(W), Y_(W), and Z_(W) between its startingand ending point. The direction numbers for U_(W) are shown in FIG. 1 asU_(Wx), U_(Wy), and U_(Wz). A second convenient representation of thedirection of U_(W) are in terms of its "direction cosines", which arethe ratios of the direction numbers to the vector length, which isconveniently equal to one for a unit vector. A third convenientrepresentation is an equation describing the plane of rotation of thewheel.

FIG. 2 is a side elevation view showing a right front wheel 20 and rightrear wheel 22. Wheel 20 rotates about its axis of rotation 21(perpendicular to the page) and wheel 22 rotates about its axis ofrotation 23 (perpendicular to the page). If the left wheels are similarto the right wheels, then a plane 24 (perpendicular to the page)contains the locations of all four wheels. For simplicity in FIG. 2, theaxes of rotation 21 and 23 are parallel and lie within plane 24. As isreadily apparent from FIG. 2, plane 24 is not necessarily parallel tothe surface 25 on which the vehicle wheels roll, due to the differencein diameters between the front and rear wheels. Line 27 is parallel tothe rolling surface 25, thereby illustrating the angle 26 between theplane 24 and the rolling surface 25.

Caster of a steerable wheel is visualized as the rearward lean, in sideelevation view, of the steering axis of that wheel. In more preciseterms as shown in FIG. 2, caster of wheel 20 is the angle 28, in sideelevation view, between the steering axis 29 and a perpendicular 30 tothe rolling surface 25. If the alignment is erroneously measuredrelative to the plane 24 as being the angle 31 between the steering axis29 and a perpendicular 32 to plane 24, then caster is measured in errorby angle 26 between plane 24 and the rolling surface 25. As is readilyapparent from FIG. 2, caster will be measured as larger than reality ifthe front wheels have smaller diameters than the rear wheels.

This error in measuring caster can have serious consequences. If casteris measured as more positive than reality (because the rear wheels arelarger in diameter than the front wheels), it would then be adjusted tobe more negative than its specification. This can lead to vehicleinstability, especially during hard braking. If caster is measured asmore negative than reality (because the front wheels are larger indiameter than the rear wheels), it would then be adjusted to be morepositive than its specification. This can lead to hard steering. It isreadily apparent from FIG. 2 that the alignment must be measuredrelative to the surface 25 on which the wheels roll, and therefore thealignment measurement apparatus must determine the locations andorientations of the wheels in relation to that surface. It is notsufficient to determine the locations and orientations of the wheelsrelative only to each other and not take the rolling surface 25 intoaccount.

FIG. 3 is a front elevation view showing a right front wheel 20 and leftfront wheel 34. Wheel 20 rotates about its axis of rotation 21 (lying inthe page) and wheel 34 rotates about its axis of rotation 35 (lying inthe page). If the rear wheels are similar to the front wheels, then aplane 36 (perpendicular to the page) contains the locations of all fourwheels. For simplicity in FIG. 3, the axes of rotation 21 and 35 arecollinear and lie within plane 36. As is readily apparent from FIG. 3,plane 36 is not necessarily parallel to the surface 25 on which thevehicle wheels roll, due to differences in the diameters between theleft and right wheels. Line 37 is parallel to the rolling surface 25,thereby illustrating the angle 38 between the plane 36 and the rollingsurface 25.

Camber of a wheel is visualized as the outward lean, in front elevationview, of the plane of the wheel. In more precise terms as shown in FIG.3, camber of wheel 34 is the angle 39, in front elevation view, betweena perpendicular 40 to the axis of rotation 35 and a perpendicular 41 tothe rolling surface 25. if the alignment is erroneously measuredrelative to the plane 36, which is not necessarily parallel to therolling surface 25, then camber is measured in error by angle 38. Thisproduces a bias in the camber measurements toward one side of thevehicle, in that camber of the wheels on one side will be measured morepositive than reality and camber of the wheels on the other side will bemeasured more negative than reality. In FIG. 3, left camber 30 and rightcamber 44 are clearly asymmetric when measured relative to the rollingsurface 25, yet these same measurements are exactly symmetric whenmeasured relative to the plane 36.

SAI of a steerable wheel is visualized as the inward lean, in frontelevation view, of the steering axis of that wheel. In more preciseterms as shown in FIG. 3, SAI of wheel 34 is the angle 42 between thesteering axis 29 and a perpendicular 41 to the rolling surface 25. Ifthe alignment is erroneously measured relative to the plane 36 as beingthe angle 43 between the steering axis 29 and a perpendicular 41 toplane 36, then SAI is measured in error by angle 38. This produces abias in the SAI measurements toward one side of the vehicle, in that SAIof the wheels on one side will be measured more positive than realityand SAI of the wheels on the other side will be measured more negativethan reality. In FIG. 3, left SAI 42 and right SAI 45 are clearlyasymmetric when measured relative to the rolling surface 25, yet thesesame measurements are exactly symmetric when measured relative to theplane 36.

The errors in measuring camber and SAI can have serious consequences. Ifcamber and SAI are measured with a side-to-side bias, then they would beadjusted with a similar bias. This can lead to "pull" and to steeringinstability. It is readily apparent from FIG. 3 that the alignment mustbe measured relative to the surface 25 on which the wheels roll, andtherefore the alignment measurement apparatus must determine thelocations and orientations of the wheels in relation to that surface. Itis not sufficient to determine the locations and orientations of thewheels relative only to each other and not take the rolling surface 25into account.

It is arguable that such errors are small, because vehicles normallyhave the same diameter tires on all four wheels, but such errors mayeasily be quite significant. Slightly different tire diameters for thefour wheels of a vehicle can arise due to many factors, such as: 1)different brands of tires, 2) different sizes of tires, 3) differenttypes of tires, 4) different tire inflation pressures, 5) different tireconditions, 6) different amounts of tread wear, and 7) differentialloading of the vehicle. For example, a 0.25 inch difference in tireradius side-to-side with a 60 inch track width produces a side-to-sidebias in camber of tan⁻¹ (0.25/60)=0.24°, thus producing a measureddifference between left and right camber of 0.48°. This is a highlysignificant error, and is in fact larger than the allowed tolerance forcamber on many vehicles.

The present invention overcomes these difficulties by referencing allthe alignment measurements to the plane on which the wheels roll. Thisis accomplished by measuring where that plane is as shown in FIG. 4.Left video camera 50 is oriented such that its field of view extendsalongside the left side of the vehicle to be measured, thus camera 50 isoutboard and above the left runway 52 of the alignment lift rack.Similarly, right video camera 51 is oriented such that its field of viewextends alongside the right side of the vehicle to be measured, thuscamera 51 is outboard and above the right runway 53 of the alignmentlift rack. The cameras 50 and 51 are mounted to opposite ends of a rigidbar 54 such that their fields of view are rigidly, though notnecessarily perfectly, aligned, each with the other. The field of viewof camera 50 looks downward and inward, such that it can see opticaltargets which are mounted to the left vehicle wheels and optical targetswhich are mounted to the left side of the left runway 52. Similarly, thefield of view of camera 51 looks downward and inward, such that it cansee optical targets which are mounted to the right vehicle wheels andoptical targets which are mounted to the right side of the right runway53. In a conventional manner, the vehicle rests on the runways 52 and 53of the alignment lift rack such that the left front wheel 55 rests onturnplate 59, the right front wheel rests on turnplate 60, the left rearwheel 57 rests on slip plate 61, and the right rear wheel rests on slipplate 62.

An optical target 64 is mounted to the right front wheel 56 such that itis within the field of view of camera 51. Similarly, optical target 63(not visible behind the curve of the wheel 55) is mounted to the leftfront wheel 55 such that it is within the field of view of camera 50.Optical target 66 is mounted to the right rear wheel 58 such that it iswithin the field of view of camera 51. Similarly, optical target 65 (notvisible behind the curve of the wheel 57) is mounted to the left rearwheel 57 such that it is within the field of view of camera 50. Theoptical targets 63-66 are rigidly mounted to the vehicle wheels 55-58 ina conventional manner such that, once mounted, their relationships withthe respective wheels are fixed throughout the measurement process, thusallowing the positions and orientations of the optical targets 63-66 tobe used to determine the positions and orientations of the wheels 55-58,respectively.

The optical targets 63-66 operate with the fields of view of cameras 50and 51 in the manner described in WO document 94/05969, except that twoseparate cameras, each with its own field of view, are used instead of asingle camera operating with beam splitters, mirrors, and such to createtwo fields of view. Using two cameras, each with its own field of view,is equivalent to using a single camera with mirrors and beam splitterssuch that the single camera has two fields of view. It is significantthat optical targets which are mounted to all four vehicle wheels cannotsatisfactorily be made to simultaneously lie in a single field of viewof a camera. It is important to note that each camera 50 and 51, i.e.each field of view, has its own coordinate system for determining thelocations of optical targets which it sees. This leads to the problem ofdetermining the relative alignment of one field of view with the other.

Cameras 50 and 51 are connected to a suitable computer and display (notshown) such that appropriate software is able to process the opticaltarget images seen by the cameras. As described in WO document 94/05969,camera 50 is used to determine, in a coordinate system relative to thefield of view of camera 50, the coordinates and orientations of theoptical targets 63 and 65, and thus the coordinates and orientations ofwheels 55 and 57, respectively. Similarly, camera 51 is used todetermine, in a coordinate system relative to the field of view ofcamera 51, the coordinates and orientations of the optical targets 64and 66, and thus the coordinates and orientations of wheels 56 and 58,respectively.

The present invention includes an optical target 67, which is mounted tothe outboard side of left runway 52 near the rearward edge of turnplate59, and an optical target 69, which is mounted to the outboard side ofleft runway 52 near the forward edge of slip plate 61. Optical targets67 and 69 appear in the field of view of camera 50. Similarly, thepresent invention includes an optical target 68, which is mounted to theoutboard side of right runway 53 near the rearward edge of turnplate 60,and an optical target 70, which is mounted to the outboard side of rightrunway 53 near the forward edge of slip plate 62. Optical targets 68 and70 appear in the field of view of camera 51. The coordinates andorientations of optical targets 67 and 69 are determined using camera 50in the same manner and at the same time as those of optical targets 63and 65. Similarly, the coordinates and orientations of optical targets68 and 70 are determined using camera 51 in the same manner and at thesame time as those of optical targets 64 and 66.

Optical targets 67 and 69 are mounted such that each is located a knowndistance from the planar surface of the runway 52. Similarly, opticaltargets 68 and 70 are mounted such that each is located a known distancefrom the planar surface of the runway 53. Thus the optical targets 67-70are used during the measurement process to define and measure thelocation in the three dimensional coordinate system of the common planeof the runways 52 and 53 on which the vehicle wheels roll. The fouroptical targets 67-70 provide redundancy, including a check to see thatthe runways 52 and 53 are adjusted such that they truly describe aplane. The manner in which this check is performed is explainedpresently.

FIG. 4 does not show the precise structure for mounting the cameras 50and 51 to the floor or to the lift rack, as such mountings are notcritical. The critical requirements are two: 1) the cameras 50 and 51must be rigidly mounted to a bar 54 or similar rigid structure, suchthat their respective fields of view are rigidly aligned, each relativeto the other, and 2) the cameras 50 and 51 and the alignment lift rackmust be stable during the measurement process such that clear,well-focused images of the optical targets are formed by the cameras.

The precise relationship between the fields of view of cameras 50 and 51is determined by a calibration procedure illustrated in FIG. 5. Acalibration stand 71 is set on the middle of left runway 52. A similarstand 72 is set on the middle of right runway 53. These stands 71 and 72are designed such that each rests on a 3-point support, thereby insuringthat the stands 71 and 72 are very solid and cannot wobble. A stiffcylindrical bar 73 is laid across the stands 71 and 72 such that it isfree to rotate through at least part of a revolution about itslongitudinal axis. An optical target 74 is mounted to the left end ofthe bar 73 such that it lies within the field of view of left camera 50.A similar optical target 75 is mounted to the right end of the bar 73such that it lies within the field of view of right camera 51. Thestands 71 and 72 and the bar 73 are designed such that no motion of thebar 73 is allowed except rotation about a longitudinal axis. The bar 73and optical targets 74 and 75 are designed such that the distancebetween the points which constitute the "locations" of the opticaltargets 74 and 75 is predetermined, and the locations of the opticaltargets 74 and 75 lie along the axis of rotation of the bar 73. Thedistance between the target "locations" in space is critical, but theorientations of the targets is not, as long as the cameras 50 and 51 cansee the targets to determine their locations and orientations.

The first phase of the calibration procedure begins by allowing thecameras 50 and 51 to see the optical targets 74 and 75 respectively, atwhich time the system determines the location and orientation of opticaltarget 74 in the field of view of camera 50 and the location andorientation of optical target 75 in the field of view of camera 51.(Note that both cameras should first be individually calibrated so thatthe various measurements are accurate. This may involve, if desired,targets such as those shown in the present application.) After thelocation and orientation of optical targets 74 and 75 are determined,the bar is then rotated through a partial revolution about itslongitudinal axis such that the optical targets 74 and 75, which rotatewith the bar, remain within the field of view of the correspondingcameras 50 and 51. The cameras 50 and 51 are again allowed to see theoptical targets 74 and 75 respectively, at which time the system againdetermines the location and orientation of optical target 74 in thefield of view of camera 50 and the location and orientation of opticaltarget 75 in the field of view of camera 51.

The change in orientation of optical target 74 is due only to therotation of the bar 73, and is therefore used to determine the equationof the line containing the axis of rotation of the bar 73 in thecoordinate system of the camera 50. Similarly, the change in orientationof optical target 75 is due only to the rotation of the bar 73, and istherefore used to determine the equation of the line containing the axisof rotation of the bar 73 in the coordinate system of the camera 51. Thetransformations required are identical to those used to determine theorientation of the axis of rotation of a wheel from the changes in theimage of the attached optical target, as described in WO document94/05969. Because the optical targets 74 and 75 are a known distanceapart, the location of target 75 is thus also known in the coordinatesystem of camera 50 and the location of target 74 is thus also known inthe coordinate system of camera 51. The relationship between the fieldof view of camera 50 and the field of view of camera 51 is thuscompletely known, allowing coordinates and orientations of targets inthe coordinate system of camera 51 to be transformed to coordinates andorientations of targets in the coordinate system of camera 50, andvice-versa.

In an alternate embodiment, the bar 73 is translated a short distancealong an axis instead of being rotated about an axis. The change inposition of the optical target 74 is due only to the translation of thebar 73, and is therefore used to determine the equation of the linedefining the axis of translation in the coordinate system of the camera50. Similarly, the change in position of the optical target 75 is dueonly to the translation of the bar 73, and is therefore used todetermine the equation of the line defining the axis of translation inthe coordinate system of the camera 51. Because the optical targets 74and 75 are a known distance apart, the location of target 75 is thusalso known in the coordinate system of camera 50 and the location oftarget 74 is thus also known in the coordinate system of camera 51. Therelationship between the field of view of camera 50 and the field ofview of camera 51 is thus completely known, allowing coordinates andorientations of targets in the coordinate system of camera 51 to betransformed to coordinates and orientations of targets in the coordinatesystem of camera 50, and vice versa.

Proper use of this (indeed of any) alignment system requires that thevehicle rest on a flat surface and the lift rack must be adjusted toprovide such a surface. Accordingly, the second phase of the calibrationprocedure begins by allowing the camera 50 to see the optical targets 67and 69 and allowing the camera 51 to see the optical targets 68 and 70,at which point the system determines the locations and orientations ofoptical targets 67 and 69 in the field of view of camera 50 and thelocations and orientations of optical targets 68 and 70 in the field ofview of camera 51. Using the known relationship between the field ofview of camera 50 and the field of view of camera 51, as determined bythe calibration procedure discussed above, the coordinates andorientations of optical targets 68 and 70 are transformed from thecoordinate system of camera 51 to the coordinate system of camera 50,such that the coordinates of all four optical targets 67-70 are known ina common coordinate system.

If the locations of the optical targets 67-70 are described by thecorresponding three-dimensional coordinates (X_(LF), Y_(LF), Z_(LF)),(X_(RF), Y_(RF), Z_(RF)), (X_(LR), Y_(LR), Z_(LR)), and (X_(RR), Y_(RR),Z_(RR)), then the locations of the optical targets 67-70 lie in a planeif ##EQU1## If, for example, three optical targets 67-69 are used todetermine a plane, then the plane is described by the equation ##EQU2##which can be expressed in the form

    Ax+By+Cz+D=0                                               (3)

The reduction to this form is made simply by evaluating the determinantand gathering like terms. The distance q (not illustrated) from therunway target 70 at (X_(RR), Y_(RR), Z_(RR)) to this plane is thendetermined by ##EQU3## The distance q is thus a direct measure of howmuch the runway located at the optical target 70 must be raised orlowered to adjust the lift rack such that the runways 52 and 53 describea flat surface. Accordingly, the distance q is displayed on the displayscreen (not shown) in numerical form or in conventional bar graph formsuch that it provides a visual aid in making these adjustments to thelift rack. It is recommended that a representative vehicle be drivenonto the runways 50 and 51 while this adjustment is made so that thelift rack is in the same loaded condition as when it is used to measureand adjust vehicles.

Further calibration information is computed to allow the calibration tobe checked during normal operation. The distance between optical targets67 and 69 is computed as ##EQU4## Also, the distance between opticaltargets 68 and 70 is computed as ##EQU5##

The calibration process is completed by storing the information obtainedduring the calibration process into a non-volatile memory. As isroutinely practiced in the art, this calibration information is thenrecalled and used as needed during normal operation.

An alternate embodiment provides that optical targets 67-70 are viewedonly during the calibration process. In the manner described above, therunways 52 and 53 are adjusted to describe a flat surface, and theequation describing that flat surface is retained in non-volatile memoryalong with other calibration data. This equation is recalled and used asneeded during normal operation. The optical targets 67-70 can betemporarily attached to or simply laid upon the runways 52 and 53 duringthe calibration process. A severe limitation of this embodiment is thatthe relationships between the cameras 50 and 51 and the runways 52 and53 must remain fixed with a high degree of precision after calibration,as any movement of the cameras 50 and 51 relative to the lift rack aftercalibration can result in a severe mismeasurement of the vehiclealignment because the plane describing the rolling surface is no longercorrectly known. A further limitation is that the calibration of thecameras 50 and 51 and the flatness of the runways 52 and 53 cannot bechecked automatically during normal operation by using the measuredlocations of the optical targets 67-70.

Determining the Alignment Parameters

During normal operation, the system operates step by step through acontinuously repeating cycle. The steps are: Step 1) The system viewsall the optical targets 63-70 using cameras 50 and 51; Step 2) Thesystem determines the coordinates and orientations of the opticaltargets 63, 65, 67, and 69 in the coordinate system of camera 50, andthe coordinates and orientations of the optical targets 64, 66, 68, and70 in the coordinate system of camera 51, using the methods andtransformations as discussed in WO document 94/05969; Step 3) Using theknown relationship between the fields of view of cameras 50 and 51, asdetermined during the calibration process, the system transforms thecoordinates and orientations of the optical targets 64, 66, 68, and 70viewed by camera 51 into the coordinate system of camera 50, such thatthe coordinates and orientations of all optical targets 63-70 are in thesame coordinate system; Step 4) As will be discussed presently, thesystem checks the calibration of the cameras; Step 5) As will bediscussed presently, the system computes the wheel alignment parametersfrom the coordinates and orientations of the optical targets 63-70; Step6) The system updates the display such that the operator may observeand/or adjust the alignment condition of the vehicle. Operating in acontinuous cyclic manner such as this is conventional.

In Step 2, several tests are routinely and automatically performed toinsure that the calibration of the system has not changed. For example,the distance between optical targets 67 and 69 is computed usingequation (5) as described previously and compared to the correspondingdistance which was determined and stored during the calibration process.A deviation indicates that the camera 50 has suffered a change in focallength and must be corrected, after which the system must berecalibrated. A similar check is performed with the coordinates ofoptical targets 68 and 70 using equation (6) to verify the focal lengthof camera 51. Additionally, the coordinates of the optical targets 67-70are checked using equation (1) as described previously to verify thatthey still lie in a plane, meaning that the runways still form therequisite flat surface. A deviation of this surface indicates eitherthat the lift rack has suffered damage or come out of adjustment, orthat the cameras 50 and 51 no longer have the same relative alignmentsof their fields of view. In either case, the system requiresrecalibration as described above. A system which passes these tests isassumed to have the same integrity as when it was last calibrated.

The alignment computations begin by determining the equation of thealignment reference plane from the locations of optical targets 67-69.This alignment reference plane is determined using equation (2) andrepresents the surface on which the vehicle wheels 55-58 roll. Note thatany three of the four optical targets 67-70 can be used for thispurpose, assuming all four targets lie in a plane. Since there are foursuch combinations possible, there are four possible equations, each inthe form of equation (3), which define the plane. These four equationscan be combined to form an "average" plane in the form of equation (3)such that its A, B, C, and D coefficients are the averages of thecorresponding coefficients of the four equations, thus "averaging" anyrelatively insignificant deviations of the runways from perfectflatness.

FIG. 6 is an expansion of FIG. 1, illustrating how the location andorientation of a vehicle wheel 10 (shown with exaggerated camber) isdetermined relative to the alignment reference plane 80. Line 81represents the rolling direction of the plane of rotation 13 of thewheel 10. It is determined as the intersection of the plane of rotation13 with the reference plane 80. The location 12 of the wheel 10 isrepresented in the reference plane 80 as point 83, which is determinedas the projection of the location 12 onto the reference plane 80.

The camber angle 78 of the wheel 10 is determined as the angle betweenthe plane of rotation 13 of the wheel 10 and a perpendicular 79 to thereference plane 80. An equivalent determination is the angle between theaxis of rotation 11 of the wheel 10 and the reference plane 80. Thecamber angle of a wheel is defined to be positive if the wheel leansoutward at the top, relative to the reference plane. In this manner, thecamber of a vehicle wheel is defined as in conventional alignmentsystems such that it is relative to the surface on which the vehiclewheels roll.

As shown in FIG. 7, the locations 84-87 of the vehicle wheels 55-58,respectively, are determined by projections onto the reference plane 80.The rolling directions 88-91 of the wheels 55-58, respectively, aredetermined by the intersections of the planes of rotations of the wheels55-58 with the reference plane 80. All the toe and symmetry vehiclewheel alignment parameters are determined from the projected wheellocations 84-87 and the wheel rolling directions 88-91 as angles ordistances in the reference plane 80.

In FIG. 7, the total front toe angle 92 is determined as the angle inthe reference plane 80 between the rolling directions 88 and 89 of thefront wheels 55 and 56, respectively. Similarly, the total rear toeangle 93 is determined as the angle in the reference plane 80 betweenthe rolling directions 90 and 91 of the rear wheels 57 and 58,respectively. Total toe of an axle is defined to be positive when theleading edges of the tires are closer together than the trailing edges.In this manner, total toe of each axle is defined as in conventionalwheel alignment systems.

In FIG. 8, line segment 94 is determined which joins the locations 84and 86, in the reference plane 80, of the left wheels 55 and 57,respectively. Similarly, line segment 95 is determined which joins thelocations 85 and 87, in the reference plane 80, of the right wheels 56and 58, respectively. The centerline 96 is determined to be the bisectorof the angle formed by line segments 94 and 95. In this manner, thecenterline 96, which is used as the reference axis for determining therear individual toe alignment angles, is defined as in conventionalwheel alignment systems.

Left rear toe 97 is determined as the angle between the rollingdirection 90 of the left rear wheel 57 and the centerline 96. Similarly,right rear toe 98 is determined as the angle between the rollingdirection 91 of the right rear wheel 58 and the centerline 96. Rearindividual toe is defined as positive when the leading edge of the tireis closer to the centerline 96 than the trailing edge. In this manner,the individual rear toe alignment angles 97 and 98 of the rear wheels 57and 58 are defined as in conventional wheel alignment systems such thateach is relative to the centerline 96 of the vehicle.

The thrust line 99 is determined as the line in the reference plane 80which bisects the angle formed by the rolling directions 90 and 91 ofthe rear wheels 57 and 58, respectively. The thrust angle 100 isdetermined as the angle in the reference plane between the thrust line99 and the centerline 96. The thrust angle 100 is defined to be positivewhen the thrust line 99 points to the right (clockwise) relative to thecenterline 96. In this manner, the thrust line 99 and the thrust angle100 are defined as in conventional alignment systems.

In FIG. 9, left front toe is determined as the angle 101 in thereference plane 80 between the rolling direction 88 of the left frontwheel 55 and the thrust line 99. Similarly, right front toe isdetermined as the angle 102 in the reference plane 80 between therolling direction 89 of the right front wheel 56 and the thrust line 99.Front individual toe is defined as positive when the leading edge of thetire is closer to the thrust line 99 than the trailing edge. In thismanner, the individual front toe alignment angles 101 and 102 of thefront wheels 55 and 56 are defined as in conventional wheel alignmentsystems such that each is relative to the thrust line 99 of the rearwheels 57 and 58.

In FIG. 10, line segment 103 is determined which joins the locations 84and 85 in the reference plane 80 of the front wheels 55 and 56,respectively. Similarly, line segment 104 is determined which joins thelocations 86 and 87 in the reference plane 80 of the rear wheels 57 and58, respectively. The front set back angle 105 is determined as theangle in the reference plane 80 between the centerline 96 and aperpendicular 106 to line segment 94. Similarly, the rear set back angle107 is determined as the angle in the reference plane 80 between thecenterline 96 and a perpendicular 108 to line segment 94. The set backangle of an axle is defined to be positive if the right wheel of theaxle is set more rearward compared to the left wheel, relative to thecenterline 96. In this manner, the front and rear set back angles 105and 107, respectively, are defined as in conventional wheel alignmentsystems such that each is relative to the centerline 96.

The track width difference angle (not shown) is determined as the anglein the reference plane 80 between line segments 94 and 95. This angleprovides an indication that the track width is not the same for thefront axle as for the rear axle. The wheelbase difference angle (notshown) is determined as the angle in the reference plane 80 between linesegments 103 and 104. This angle provides an indication that thewheelbase is not the same for the left side of the vehicle as for theright side. In this manner, the track width difference angle andwheelbase difference angles are defined as in conventional wheelalignment systems.

In FIG. 10, the wheelbase 111 at the left side of the vehicle isdetermined as the distance, in a direction parallel to the centerline96, between the left wheel positions 84 and 86, respectively. Similarly,the wheelbase 112 at the right side of the vehicle is determined as thedistance, in a direction parallel to the centerline 96, between theright wheel positions 85 and 87, respectively. The track width 113 atthe front end of the vehicle is determined as the distance, in adirection perpendicular to the centerline 96, between the front wheelpositions 84 and 85, respectively. Similarly, the track width 114 at therear end of the vehicle is determined as the distance, in a directionperpendicular to the centerline 96, between the rear wheel positions 86and 87, respectively. In this manner, the track width and wheelbasemeasurements are defined as in conventional alignment systems.

The caster and SAI angles of the steerable wheels are determined as inconventional alignment systems per the procedure discussed in SAEPublication 850219, titled "Steering Geometry and Caster Measurement",by January.

Other optical targets can be mounted to the vehicle frame, vehicle body,or some combination thereof. The locations of these targets, which canbe determined in the same manner as the targets mounted to the vehiclewheels and to the runways, can be projected onto the reference plane andused to determine the longitudinal reference axis for measuring rearindividual toe and other parameters. They can also be used to measurewheel and axle offset and set back, in the same manner as described inU.S. Pat. No. 5,488,472. The difficulty with this, however, is thatthere are no industry standards for mounting such targets to the vehiclebody or frame or for making use of the location measurements thereof.

Turning to FIGS. 11 and 12, there is shown the present invention in usewhere the surface upon which the wheels rest are the runways of anautomotive lift 121. In this embodiment, it is preferred that cameras 50and 51 (or single camera 50 if a single camera system is used) beconnected to a suitable elevating mechanism 123 so that the opticaltargets maintain their same position in the field of view of the cameraswhen the runways and vehicle are elevated. In this embodiment thecomputer, labeled 125, is used to control the camera elevating mechanism123 so that as the automotive lift runways are raised, the cameras areraised the corresponding amount. (Of course, control mechanisms otherthan the computer could be used as well). Many mechanisms are known forsuitably elevating the cameras, such as hydraulic post systems, jackscrews with motors, rack and pinion systems, and the lack.

Camera position control can be exercised in a number of ways. Forexample, as shown by an optional lift position sensor 131 in FIG. 12,the lift position information can be fed directly to computer 125, whichin turn controls the camera elevating mechanism 123 to move thecamera(s) a corresponding amount so that the targets remains in the sameposition in the field of view of the camera(s). Alternatively, thecameras can be mechanically coupled to the elevating mechanism(indicated by dashed line 141) such that said first and second videocameras are moved equal vertical distances by the elevating mechanism.

Similarly, computer 125 can be programmed to be responsive to theposition of at least one target in the field of view to control theelevating mechanism 123 to move the video camera(s) so as to maintainthat target at a predetermined position in the field of view. With thissystem, computer 125 can, by observing variation in position of theoptical targets in the corresponding field of view, detect misalignmentof the runways. It is preferred that lift 121 further include apparatusunder the control of the computer (such as separate lift posts for eachrunway) for adjusting the geometrical orientation of the runways inresponse to the detected misalignment.

For some applications, it may be desirable for the elevating mechanism123 to be manually actuable, with the computer merely signalling to theoperator when the targets are in the proper position in the field ofview.

Although the present invention has been described in terms of keepingthe fields of view of the cameras constant as the vehicle is elevated,it should be understood that in many instances the vehicle may belowered to floor level from a standard 30" level to allow work to bedone. It is anticipated that the present invention shall move thecameras correspondingly when the vehicle is lowered as well as when thevehicle is elevated.

In view of the above it will be seen that the various objects andfeatures of the invention are achieved, and other advantageous resultsobtained. It should be understood that the description contained hereinis illustrative only and is not to be taken in a limiting sense.

What is claimed is:
 1. A wheel alignment apparatus for determining thealignment of the wheels of a vehicle, said apparatus comprising:a set ofpredetermined optical targets adapted to be mounted in positions toprovide images functionally related to wheel alignment parameters; atleast one video camera disposed to receive images of said opticaltargets, each of said at least one video cameras having a field of view;a computer operatively connected to said at least one camera, saidcomputer being responsive to the images of said set of targets todetermine values of wheel alignment parameters of the vehicle; and anelevating mechanism operatively connected to the at least one videocamera to raise and lower said at least one camera along a predeterminedpath, said elevating mechanism being responsive to vertical movement ofthe vertically movable surface to move said at least one cameracorrespondingly along said predetermined path.
 2. The wheel alignmentapparatus as set forth in claim 1 further including a vertically movablesurface comprising the runway of a lift rack on which the vehicle isdisposed, said elevating mechanism being responsive to vertical movementof the lift rack from a first vertical position to a second verticalposition to move said at least one camera along the predetermined pathsuch that said first set and said second set of predetermined opticaltargets of the lift rack remain substantially in the same positions inthe field of view of said at least one camera.
 3. The wheel alignmentapparatus as set forth in claim 2 wherein the at least one video cameraincludes first and second video cameras, said first and second videocameras being mechanically coupled to the elevating mechanism such thatsaid first and second video cameras are moved equal vertical distancesby the elevating mechanism.
 4. The wheel alignment apparatus as setforth in claim 3 wherein the first and second video cameras are disposedin a fixed geometrical relationship.
 5. The wheel alignment apparatus asset forth in claim 2 wherein the computer is responsive to the positionof at least one target in the field of view to control the elevatingmechanism to move the video camera along the predetermined path so as tomaintain said at least one target at a predetermined position in thefield of view.
 6. The wheel alignment apparatus as set forth in claim 5wherein said at least one video camera includes first and second videocameras disposed to view opposite sides of the vehicle, at least oneoptical target being in a field of view of the said first video cameraand at least one optical target being in a field of view of the secondvideo camera.
 7. The wheel alignment apparatus as set forth in claim 6wherein the computer is responsive to variation in position of one ofthe optical targets in the corresponding field of view to detectmisalignment of the runways, further including means for adjusting thegeometrical orientation of the runways in response to the detectedmisalignment.
 8. A wheel alignment apparatus for determining thealignment of the wheels of a vehicle, said apparatus comprising:a set ofpredetermined optical targets adapted to be mounted to wheels of avehicle; at least one video camera disposed to receive images of saidoptical targets, each of said at least one video cameras having a fieldof view; a computer operatively connected to said at least one camera,said computer being responsive to the images of said set of targets todetermine values of wheel alignment parameters of the vehicle; and anelevating mechanism to raise and lower said at least one camera along apredetermined path an amount such that said predetermined opticaltargets remain substantially in the same position in the field of viewof said at least one camera.
 9. The wheel alignment apparatus as setforth in claim 8 wherein the elevating mechanism is responsive tomovement of a vertically movable surface on which the vehicle isdisposed to automatically move said at least one video camera acorresponding amount along the predetermined path.
 10. The wheelalignment apparatus as set forth in claim 8 wherein the elevatingmechanism is manually operable.